Fluorescent lights are the most common light source used in commercial buildings. As is well known, to provide the same amount of light output, a fluorescent bulb requires more electrical power when cold than after it has heated up. Once heated up, the extra power provided during start-up is dissipated as heat and does not substantially affect the light output of the bulb as perceived by the user. Indeed, the extra power has the advantage that the light output from the bulb does not perceptibly decrease when the line voltage drops, as, for example, when a large piece of electrical equipment such as an air-conditioner compressor comes on-line.
As a result of these considerations, the manufacturers of fluorescent lighting have historically used ballasts whose effective impedance, whether provided by electrical components (resistors, capacitors and/or inductors) or by electronic circuits, has been selected so that the fluorescent bulb is always operated in an overpowered condition. This overpowering has been substantial, with power consumption typically running 20-30% higher than that actually required to operate a warmed-up fluorescent bulb. Recently, energy efficient fluorescent bulbs have been introduced to the market. These bulbs consume less energy but take substantial periods of time before they reach full light output. Also, these energy efficient bulbs come with their own ballasts and thus do not address the problem of the installed base of existing fluorescent fixtures with ballasts designed for overpowering.
The need to reduce the electrical power consumed by lighting has become especially acute in older buildings whose electrical systems were designed at the time when the use of electronic equipment, e.g., computers, was less prevalent or substantially non-existent. In many of these buildings, it is physically impossible to feed more power to the building, especially in the case of older skyscrapers where the electrical conduits running up the center of the building are already filled to capacity. The situation has become so severe in some older buildings that entire floors cannot be used because of insufficient power to both light the floor and provide users with the electricity needed in a modern office.
Some efforts have been made to address the power consumption problem by installing fixed phase SCRs in the power lines leading to fluorescent light fixtures. In practice, these approaches have proved unacceptable to consumers because of (1) low output from the fluorescent lights during start-up and (2) reduced light output under low voltage conditions. That is, these approaches have simply re-introduced the problems which led to overpowering in the first place. Moreover, switching of the SCRs leads to unacceptable levels of noise (e.g., harmonic distortion) on utility power lines, as well as power factor issues. For large buildings, the electrical noise/power factor problems can reach levels where power companies often seek premium rates for their power, thus reducing the consumer's economic incentive to reduce power consumption.
There thus exists a need in the industry for improved systems for reducing the power consumption of fluorescent lights. In particular, there exists a need for improved systems that can be readily retrofitted into existing buildings employing overdriven fluorescent lights without reintroducing start-up and low voltage problems and without significantly affecting the operation of utility power lines. The present invention, in its preferred embodiments, addresses and solves these existing problems in this field.